A sexually selected character displacement in flycatchers reinforces premating isolation

Theory suggests that natural selection against the production of unfit hybrids may reinforce barriers to gene flow, eventually leading to reproductive isolation of differentiated populations. This mode of speciation may be achieved by female choice selecting for a divergence in male secondary sexual traits that facilitates species recognition. Although intuitively appealing, conclusive evidence for such reinforcement is generally lacking, and serious doubts have been raised about its validity. We have tested key predictions of the reinforcement hypothesis on the European, black-and-white, Ficedula flycatchers, using molecular techniques, field observations and mate choice experiments. In populations where two species coexist, we show that female choice selects for a divergence in male plumage colour and that the resulting character displacement reduces the frequency of hybridization.